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Google App Engine compliant variation of FreeMarker. FreeMarker is a "template engine"; a generic tool to generate text output based on templates.

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/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package freemarker.template.utility;

import java.io.IOException;
import java.io.Writer;
import java.util.Map;

import freemarker.template.TemplateTransformModel;

/**
 * Performs an HTML escape of a given template fragment. Specifically,
 * < > " and & are all turned into entities.
 *
 * Usage:

 * From java:
 *  * SimpleHash root = new SimpleHash();
 *
 * root.put( "htmlEscape", new freemarker.template.utility.HtmlEscape() );
 *
 * ...
 * 
 *
 * From your FreeMarker template:
 *  *
 * The following is HTML-escaped:
 * <transform htmlEscape>
 *   <p>This paragraph has all HTML special characters escaped.</p>
 * </transform>
 *
 * ...
 * 
 *
 * @see freemarker.template.utility.XmlEscape
 */
public class HtmlEscape implements TemplateTransformModel {

    private static final char[] LT = "<".toCharArray();
    private static final char[] GT = ">".toCharArray();
    private static final char[] AMP = "&".toCharArray();
    private static final char[] QUOT = """.toCharArray();

    @Override
    public Writer getWriter(final Writer out, Map args) {
        return new Writer()
        {
            @Override
            public void write(int c)
            throws IOException {
                switch(c)
                {
                    case '<': out.write(LT, 0, 4); break;
                    case '>': out.write(GT, 0, 4); break;
                    case '&': out.write(AMP, 0, 5); break;
                    case '"': out.write(QUOT, 0, 6); break;
                    default: out.write(c);
                }
            }

            @Override
            public void write(char cbuf[], int off, int len)
            throws IOException {
                int lastoff = off;
                int lastpos = off + len;
                for (int i = off; i < lastpos; i++) {
                    switch (cbuf[i])
                    {
                        case '<': out.write(cbuf, lastoff, i - lastoff); out.write(LT, 0, 4); lastoff = i + 1; break;
                        case '>': out.write(cbuf, lastoff, i - lastoff); out.write(GT, 0, 4); lastoff = i + 1; break;
                        case '&': out.write(cbuf, lastoff, i - lastoff); out.write(AMP, 0, 5); lastoff = i + 1; break;
                        case '"': out.write(cbuf, lastoff, i - lastoff); out.write(QUOT, 0, 6); lastoff = i + 1; break;
                    }
                }
                int remaining = lastpos - lastoff;
                if (remaining > 0) {
                    out.write(cbuf, lastoff, remaining);
                }
            }
            @Override
            public void flush() throws IOException {
                out.flush();
            }

            @Override
            public void close() {
            }
        };
    }
}